Ethylene is oxidized under the action of the silver catalyst to mainly produce ethylene oxide, which is accompanied by the side reaction to produce carbon dioxide and water. The main technical properties of the silver catalyst include activity, selectivity and stability. The activity is reflected by the reaction temperature at which ethylene oxide is produced with a certain reaction load. The lower the reaction temperature is, the higher the catalyst activity is. The selectivity means the ratio of the mole of ethylene that converts to ethylene oxide to the total mole of ethylene that takes part in the reaction. The stability is reflected by the rate at which the activity or selectivity drops. The lower the dropping rate is, the better the catalyst stability is. The silver catalyst having high activity, high selectivity and good stability during the production of ethylene oxide by the oxidization of ethylene can improve the economical benefit remarkably. Thus, it is a main research object of the silver catalyst to make a silver catalyst having high activity, high selectivity and good stability. The property of the silver catalyst has a great relation with the property of the support used in the silver catalyst preparation and the preparation method of the support besides the composition of the catalyst and the preparation method of the catalyst.
The silver catalyst preparation method in the prior art comprises the steps of preparing porous support such as alumina and applying active components and promoters onto said porous support.
It is an important research aspect to add some components to alumina support to modify the support so as to improve the silver catalyst property. In this connection, alkaline-earth metal oxides or other salt compounds are added to the support.
U.S. Pat. No. 4,428,863 discloses a small amount of binder such as barium aluminate or barium silicate is used in the production of the alumina support having a high purity and a low surface area. It is alleged that the crushing strength and the wear resistance of the support can be improved. The prepared support has a specific surface area of less than 0.3 m2/g, as well as low activity and low selectivity.
U.S. Pat. No. 5,384,302 alleges that the pre-treatment of alpha-alumina to reduce Na, K, Ca and Al ion contents in the support can improve the crushing strength and the wear resistance of the support.
U.S. Pat. No. 5,739,075 discloses a promoting-amount of rare-earth metal and a promoting-amount of another metal salt (alkaline-earth metal or Group VIII transition metal) is pre-doped on the surface of alumina support, followed by calcination, and then the treated support is used to produce the silver catalyst. The evaluation result shows that the dropping rate of the catalyst is lower than that of the catalyst without the pre-doping treatment.
Fluoride is a mineralizing agent and widely used in the production of alumina support.
CN Patent Publication No. 1034678A discloses mixing appropriate sizes and amounts of trihydrate alpha-alumina and pseudo-boehmite with carbonaceous material, fluxing agent, fluoride, binder and water, kneading into shape, drying, and calcining to produce the alpha-alumina support. The support has a specific surface area of 0.2-2 m2/g. Pores with the radii of more than 30 μm comprise 25-10% of the total pores. The support is impregnated with silver compound and promoters, dried and activated to be used in the production of ethylene oxide by the oxidation of ethylene with a selectivity as high as 83-84%.
CN Patent Publication No. 101007287A discloses mixing certain sizes of trihydrate alpha-alumina and pseudo-boehmite with certain amounts of burnout carbonaceous material, fluxing agent, fluoride, and optional heavy alkaline-earth metal compound homogenously, followed by adding binder and water, kneading homogenously, extruding into shape, drying, and calcining to produce the alpha-alumina support. The support has a specific surface area of 0.2-2 m2/g, a pore volume of 0.35-0.85 ml/g, water absorption of ≧30%, and a crushing strength of 30-120 N/particle. This support is impregnated with a solution of silver-amine complex, alkali metal compound and alkaline-earth metal compound, dried and activated to produce a silver catalyst useful for the production of ethylene oxide by the oxidation of ethylene.
CN Patent Publication No. 1634652A discloses a process for preparing the support without using a pore-forming agent. In that process, trihydrate alpha-alumina is directly mixed in a certain proportion with pseudo-boehmite, fluxing agent, and fluoride homogenously, added binder and water, kneaded homogenously, extruded into shape, dried, and calcined to produce the alpha-alumina support. The support has a specific surface area of 0.2-2.0 m2/g, a pore volume of 0.35-0.85 ml/g, water absorption of ≧30%, and a crushing strength of 20-90 N/particle. This support is impregnated with a solution of silver-amine complex, alkali metal compound and alkaline-earth metal compound, dried and activated to produce a silver catalyst useful for the production of ethylene oxide by the oxidation of ethylene.
Albeit the above prior arts suggest adding alkaline-earth metal compound and fluoride to alumina starting material so as to modify alumina support and further to produce the catalyst with good activity and selectivity, there is still a continuous demand for the alumina support having better properties. According to the present invention, a potassium melt technology is adopted to prepare an alumina support, from which a silver catalyst with a higher selectivity can be made.